Gas bubble elimination in liquid-cooled electrical apparatus



K. K. PALUEV Oct. 21, 1952 GAS BUBBLE ELIMINATION IN LIQUID-COOLED ELECTRICAL APPARATUS Filed Oct. 16, 1946 PUMP.

Pig. 6.

Inventor-z Konstantin K. aluev,

His Attorney.

Patented Oct. 21, 1952 GAS BUBBLE ELIMINATION IN LIQUID- COOLED ELECTRICAL APPARATUS Konstantin K. Paluev, Pittsfield, Mass, assignor to General Electric Company, a corporation of New York Application October 16, 1946, Serial No. 703,540

4 Claims. 1

This invention relates to the elimination of gas bubbles in liquids and more particularly to improvements in forced liquid cooled electrical apparatus.

It is well known that when liquid is withdrawn from a container through an orifice below the level of the liquid, a vortex will often form between the surface of the liquid and the orifice. Sometimes, this vortex will extend all the way down from the level of the liquid to the orifice in which case if there is gas or vapor above the liquid level, bubbles of this gas or vapor may be drawn into the orifice. In general, for a given rate of flow through the orifice, the formation of a vortex is determined by the height of the liquid level above the orifice and there is usually a critical height above which no vortex will form and below which a vortex will always form.

In certain electrical apparatus such, for example, as transformers, it is well known practice to place the apparatus in a casing which is substantially filled with a dielectric liquid and to cool such apparatus by withdrawing liquid from near the level thereof forcing it through a suitable cooler and returnin it to the tank near the bottom thereof. It is also well known practice to force the cooled liquid into winding ducts in the apparatus so as to provide more eflicient cooling. In the construction of large transformers, severe limitations are imposed by a number of factors among which are highway and railroad bridge clearances, on the height of such apparatus. This means that in the larger size transformers, it is not always possible to have sufficient distance between the liquid level and the orifice through which liquid is discharged from the transformer tank to insure that a vortex will not form.

The formation of a vortex in high voltage electrical apparatus of the above-described type can easily be disastrous when it is realized that the dielectric strength of gas is negligible compared to the dielectric strength of conventional dielectric liquids such as mineral oil and askarel. In other words, if a gas bubble following a vortex should find its way into a winding duct where the potential gradients are usually quite high, an arc would be established which would quickly destroy the apparatus.

I have found that the formation of vortices can be eliminated by the use of a solid (nonfluid) surface which is placed in the liquid between its level and the discharge or outlet orifice. This therefore, makes it possible materially 2 to reduce the maximum height of induction apparatus as the height of level above the discharge orifice need not be maintained at any particular minimum value as heretofore. I have found that very good results are obtained by the use of a screen as the solid surface which is used to prevent the formation of a vortex.

Another cause of gas bubble formation which I have discovered is the reduction in fluid pressure on the suction side of the circulating pump. This pressure reduction permits gas which has been absorbed by the liquid to come out of solution and form into bubbles. One way of preventing this from occurring is to correlate the location and suction of the pump that the velocity head of the liquid is less than its static head at the pump inlet or suction side.

An object of the invention is to prevent the presence of gas bubbles in forced liquid cooled electrical apparatus.

Another object of the invention is to prevent gas which has been absorbed in cooling liquid for electrical apparatus from forming into bubbles below the surface of the liquid.

An added object of the invention is to eliminate vortices in liquids.

An additional object of the invention is to reduce the height of electrical induction apparatus.

A further object of the invention is to prevent gas from the top of a container from entering forced liquid cooled electrical apparatus.

A still further object of the invention is to prevent the formation of vortices at the top'of the cooling liquid in forced liquid cooled transformers having a winding duct.

The invention will be better understood from the following description taken in connection with the accompanyin drawing and its scope will be pointed out in the appended claims.

In the drawing, Fig. lis a sectional view of a transformer embodying one form of my invention; Fig. 2 is a part sectional view on line 22 of Fig. 1 while Figs. 3, 4, 5 and 6 illustrate modifications of the invention.

Referring now to the drawing and more par ticularly to Fig. 1, there is shown therein a transformer comprising a tank I substantially filled with liquid to the level 2 in which is immersed a magnetic core 3 and a pair of windings 4 and 5 surroundin the core. Between the core 3 and the inner Winding 4 is a cooling duct 6 and between the windings themselves is another cooling duct 1.

For cooling the apparatus. there is provided a pump 8 of any suitable type and a cooler :9 in

the form of a tubular radiator. The liquid is withdrawn by the pump 8 from below the level 2 through an orifice l and is drawn through the cooler 9 and forced back into the tank I through a conduit ll. Suitable baffles [2 are provided for insuring that the cooled liquid from the conduit II will be forced into the lower ends of the ducts 6 and I.

For preventing the formation of a vortex in the liquid and for preventing a substantial reduction in the permissible distance between the level 2 and the orifice it, a metal mesh or screen 13 is provided below the surface of the liquid and above the orifice ill. As shown in Fig. 2. this screen may be of rectangular shape having diamond-shaped openings.

Investigation has shown that in certain transformer designs, it is necessary to keep the liquid level 16 or 17 inches above the top point of the discharge orifice in order to prevent the formation of a vortex when no other means is provided for preventing its formation. However, when such a transformer is provided with a rectangular screen [3 which is about 12 inches long and 8 inches wide and which is placed about one-half inch below the liquid surface over the orifice with the long edge in contact with the tank wall, it was found that no vortex would form with the liquid level 3 inches above the orifice.

By placing the pump 8 below the cooler 9,

instead of above it, the static head of the liquid between the conduits i9 and II is available for maintaining fluid pressure on the inlet or suction side of the pump. By so proportioning the size of the pump that the velocity head of the liquid as it is drawn into the pump is less than the static head of the liquid on the suction side of thepump a positive liquid pressure will always exist on this side of the pump and in this way there will be no more tendency for absorbed gas to form bubbles at the inlet side of the pump than at a point just below the surface of the liquid.

In certain apparatus, the liquid level will not remain constant for various reasons, such as its thermal expansion and contraction, or loss due to leaks. In such case, it is preferable to provide a plurality of vertically spaced screens I3, l3, and [3, as shown in Fig.3.

' Another way of compensating for the variation in liquid level is shown in Fig. 4. This comprises mounting screen l3 on a float Mso that the screen will always stay the same distance below the liquid level.

In another modification, shown in Fig. 5, a cup or dish shaped screen 115 is placed over the orifice ID.

The use of screens is not essential and imperforate parallel surfaces such as are shown at IS in Fig. 6 may be employed.

While there have been shown and described embodiments of this invention, it will be obvious to those skilled in the art that changes and modifications can be made without departing from the invention and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention. In this connection, it is pointed out that bubbles may be produced in the liquid by a surface vortex which entraps some of the gas of the free gas space which is above the surface of the liquid or by evolving gas due to reduction of the pressure in the liquid below the pressure Pb of gas which is dissolved or absorbed in the liquid.

The pressure in the liquid at any pointis the 4 algebraic sum of several partial pressures. Among these are Pv the negative pressure due to rotation of the liquid, as in a vortex. However, there are two different kinds of vortexes, namely, surface and submerged. In a surface vortex, there is usually a gas filled cone extending down into the liquid from its surface. Not only can such a surface vortex entrap gas but it can also evolve gas due to a reduction of liquid pressure caused by rotation of the liquid. As the minimum pressure in a surface vortex is at the apex, it is natural for gas to evolve first there. A submerged vortex has no gas filled cone but simply consists of a mass of liquid in a state of rotation. A screen like IS in the drawing is effective in preventing development of a vortex of either type reaching a dangerous intensity. However, it is natural that the location of the screen required by a submerged vortex may be different from that required by a surface vortex. In general, to use a screen for suppression of submerged vortexes, it should be located in the neighborhood of their generation which may be anywhere along the path of the flowing liquid.

Another partial pressure in the liquid is Pd which is a negative pressure due to the flow or velocity of the liquid. In a hydraulic system like the one described above, the pressure of the fluid due to its flow is continuously reduced from the intake I 0 to a certain point in the pump like, for example, at the surface of the impeller. Because of this, gas may be evolved anywhere along a stream of liquid from the intake ID to the point of lowest pressure and not in the pump only. For example, by having a valve at the intake 10 of Fig. 1, it is possible to cause bubble formation at the valve if it is closed sufficiently.

Other partial pressures of the liquid are Pg which is the gas pressure above the surface of the liquid, PS which is the pressure due to the static head of the liquid and Pt which is the pressure due to the surface tension of the liquid. All these are positive, i. e. they oppose formation of bubbles in the liquid. Surface tension acts somewhat as a bag made of the liquid. For bubbles to form, the gas must inflate the bag. The surface tension of the liquid in the presence of a given gas is a measure of the resistance Pt this bag offers to its inflation.

where T is surface tension per unit and D is the diameter of the bubble.

The above may be summarized by the formula that Pb must be less than Pg plus P5 plus Pt minus Pv minus Pd.

While in Fig. l, the flow of liquid is up through the winding ducts 6 and l and down through the cooler 9, I may also choose the opposite direction as more advantageous from the point of view of suppression of the evolution of gas from the liquid. Such advantage is obtained where the dynamic pressure drop Pd required to sustain the desired flow through the winding ducts is lower than that required by the radiator and other parts of the heat exchanger. The advantage comes from the fact that the smaller is the dynamic pressure Ps on the suction side of the pump, the smaller is the static head required to prevent evolution of gas for a given gas pressure and therefore a greater vortex pressure Pv is allowed. It follows that I can use either a lower tank or a higher level for the pump or .less care in the shaping and proportioning of the interior of the suction side of the system than otherwise would be necessary for the same margin of safety in regard to the formation of bubbles.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In combination, a tank, a liquid in said tank, a discharge orifice for said liquid in said tank below the level of said liquid, a float in said liquid, and metal screen supported by said float in said liquid above said orifice for preventing the formation of a vortex.

2. In combination, encased liquid filled induction apparatus having a winding duct, a cooler, means including a pump for withdrawing liquid near the level thereof from said apparatus, forcing it through said cooler and returning it to said apparatus so that it passes through said duct, a float in said liquid, and a mesh screen supported by said float in said liquid between its level and its pointof withdrawal for preventing formation of a vortex in said liquid.

3. In combination, encased liquid filled induction apparatus having a winding duct, a cooler, means including a pump for withdrawing liquid near the level thereof from said apparatus, forcing it through said cooler and returning it to said apparatus so that it passes through said duct, a float in said liquid, and a mesh screen supported by said float in said liquid between its level and its point of withdrawal for preventing formation of a vortex in said liquid, said pump being so located that said liquid provides an appreciable static pressure head on the suction side thereof, said pump having a velocity pressure head which is less than said static pressure head.

4. In combination, encased liquid filled induction apparatus having a Winding duct, a cooler,

KONSTANTIN K. PALUEV.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,232,834 Nichols July 10, 1917 1,761,580 Nichols June 3, 1930 1,835,470 Clarke Dec. 8,1931 1,887,569 St. Palley Nov. 15, 1932 2,161,031 Fletcher June 6, 1939 2,316,763 Kerr Apr. 20, 1943 FOREIGN PATENTS Number Country Date 293,108 England June 29, 1928 OTHER REFERENCES Worthington Pump Handbook, 1st Ed. (1927), See. VI, page 9.

Stepanoff, A. J. Trans. 1945, page 541.

A. S. M. FL, October 

